Dear Mr. Electrician: How do I install an electrical receptacle box in a wall using Madison Bars? I want to install an electrical outlet on a dedicated circuit for my window air conditioner and someone told me to use Madison Bars to mount the outlet box.
Answer: Madison Bars are a simple method for installing a gangable metal outlet box in a wall. They are also known as Old Work Box Mounts, Support Clips, F Clips, Madison Clips, and Madison Straps. The photo above shows Madison Bars that are two different lengths. The longer ones are for working in walls that have double drywall or a finished surface such as tile or paneling that makes the wall board thicker.
In addition screw clips are shown that can also be used to attach metal outlet boxes to wall board. They just clip on with a little force. Once the box is in the wall, you tighten the screw and the clips wedge themselves between the box and the inside of the wall. You can also buy some outlet boxes with the clips built-in.
In the example below the hole was made using a Compass saw. The metal outlet box was held against the wall and a pencil line was traced onto the surface. Note the notches on the upper right and lower left corners. They are to allow for the protruding screws on each side that all gangable metal receptacle boxes of this type have. NOTE: Text links below go to applicable products on Amazon.com
The boxes are gangable by removing one or both sides and screwing the boxes together with additional boxes to make a multi-gang switch or outlet box. You can add as many gangs as needed, however the more gangs, the more difficult it will be to find wall plates to cover the finished work. You can special order larger wall plates from an electrical supply company, but you may be limited in the choice of colors and styles.
For the hole above I held the face of the electrical box against the wall and penciled an outline to follow with my Compass Saw.
Installing an Electrical Receptacle Outlet Box using Madison Bars
The ground wire above is attached to the box using a separate 10-32 ground screw. Consult Article 314 in the National Electrical Code for the correct size electrical box needed for the amount of wires and devices that will be contained within.
It is important to have the hole cut exactly right. If the hole is too big the ears on the electrical box will not hold against the wall as the Madison Bars will pull the box inward.
The ideal location for the new outlet box is next to a wood wall stud. In that scenario I will drill a 3/16″ hole about midway back on the side of the outlet box that will be against the wood stud. After I install the Madison Bars securely I will screw a #8 x 1 1/4″ sheet metal screw into the wood stud. Do not over tighten the screw as it will distort the box and make it tilted.
When mounting the box adjacent to a wood stud, it is best to notch the wood stud a little where the protruding screw tab on the box will come in contact with the wood. This is done to keep the box straight otherwise the protruding screw tab will cause the box to tilt.
I used to use my compass saw to make the notch or sometimes a wood chisel and a hammer. However since I got my oscillating multi-function power tool I just use that. The photo below shows the notch in the wood wall stud in the lower left.
In the photo above the notch in the wood stud is to accommodate the protruding screw on the box is shown on the lower left.
All extra cable clamps must be removed, because each one counts as one additional wire when calculating box fill. See article 314.16 and tables 314.16(A) and 314.16(B) in the National Electrical Code.
Bring the cable into the electrical box and tighten the clamp enough so that the cable cannot be easily pulled out. Do not over-tighten the clamp as it could cause an internal short circuit on the cable. Remove the other unused clamp entirely and save the screw for grounding. In the photo below all extra clamps have been removed and one 10/32 machine screw has been inserted for grounding purposes.
The box in the photo above is attached to the wall with one Madison Bar and one #8 sheet metal screw. A 10/32 screw for grounding is in the bottom right side.
Wrap the ground wire around the 10/32 ground screw clockwise and tighten the screw so that the metal electrical box will be grounded.
With the cable clamped you can push the electrical box into the wall and insert one Madison Bar along side of the box. Slide up the long side so that the short side of the Madison Bar can go into the wall. With the opposite side of the electrical box protruding about 1/4″ – 3/8″ push the side of the box with the Madison Bar into the wall hard using a big flat head screwdriver. While holding the one edge of the box in with the large screwdriver, bend the metal tabs of the Madison Bar inward towards the inside of the box. Squeeze the tabs tightly with long nose pliers.
It is important to have the opposite side of the outlet box protrude out a little. That will make the Madison Bar on that side very tight and will hold the box in the wall more securely.
You can use a large flat head screwdriver to push the box against the wall and hold it there, the bigger the easier. This ensures that the box will be held in place tightly. Use needle nose pliers to crimp the Madison Bars tightly against the metal electrical box.
Insert another Madison Bar on the other side of the electrical box with the long end of the Madison Bar pointing in the opposite direction of the first Madison Bar. Push the box in with the large flat head screwdriver and bend the metal tabs over. Pinch the tabs tightly with needle nose or with long nose pliers.
Once the Madison Bars are fully in place, the electrical box should be very snug against the wall. It should not be loose. Note how the upper and lower screw hole tabs on the box are set back into the wall slightly. Inside the rear of the box is the bare copper ground wire attached to the metal box using a 10/32 screw.
After the box is secure you can adjust it for straightness by lightly tapping it with a screwdriver and your pliers.
Remove the ears when installing a wiring device such as a switch or electrical receptacle outlet onto a gangable metal box that has its ears on the wall surface. The wall plate will fit better against the wall. In the photo below I removed the ears, but neglected to get a photo of the GFCI receptacle without them.
I prefer to install the old work electrical box next to a wall stud so that a wood or sheet metal screw can be screwed through a hole in the box directly into the wood stud in addition to the Madison Bars. That makes the box extremely secure. However on this job the client wanted the receptacle installed in the exact location above.
The photo below shows the wiring devices with the ears removed so that they sit closer to the wall. The copper colored clip on the bottom of the GFCI outlet is for self-grounding. This means that the outlet is grounded through the screw and no additional grounding conductor is necessary. However I still connect the grounding conductor to the outlet to ensure a good grounding connection.
The ears on the outlets above have been removed so that the outlets seat better on the electrical box. A cord is plugged into the GFCI protected outlet.
The Madison Bars tend to work best with drywall that is at least 1/2″ thick or thicker. I sometimes had a problem using Madison Bars on 3/8″ drywall. That is why it is always best to mount a new box next to a wall stud. That gives you something to screw on to in case the Madison Bars don’t hold well. Many times I will drive one #8 sheet metal screw through the side of the electrical box box anyway just to keep it secure.
The image below depicts a metal outlet box with a Madison Bar on the right and a #8 sheet metal screw on the left. Sometimes I don’t need two Madison Bars.
You can see in the above photo the 12/2 Type NM cable is entering from the bottom under the cable clamp. The upper cable clamp has been removed and the clamp screw has been reinstalled for grounding the box.
Below you can clearly see the #8 sheet metal screw on the left going into a wood stud. The screw must be set back enough so that the screw head does not come in contact with the receptacle. I usually drill my hole in the middle of a 3.5″ deep gangable outlet box. I use an awl to start the hole so the screw catches easily and it is at a good angle for me to screw it in.
Click here for my post about installing an electrical receptacle. It will be helpful.